Liquid crystal display panels are in the widest use today as thin and lightweight display devices. A liquid crystal display panel is a light valve for controlling the voltage applied to a liquid crystal layer by a switching element such as a thin film transistor or an MIM (metal-insulator-metal) element on a pixel-by-pixel basis and thus adjusting the amount of light transmitted through the liquid crystal layer. The liquid crystal display devices are not self-light emission elements which emit light themselves and thus generally have problems of dark images and narrow viewing angles.
As a thin and lightweight self-light emission element solving such problems of the liquid crystal display devices, an electron &mission element has been a target of attention. The electron emission element is not a hot-electron emission type element for heating a cathode to emit electrons as a conventional CRT, but is a cold cathode type element for extracting electrons from the cathode by electric field.
Regarding the conventional electron emission elements, for example, a technology for producing a micrometer-size fine vacuum element utilizing a microscopic processing technology used for producing semiconductor transistors and the like (see, for example, (1) Junji ITO, Oyo Buturi, Vol. 59, No. 2, pp. 164-169, 1990, or (2) Kuniyoshi YOKOO, Journal of the Institute of Electrical Engineers of Japan, Vol. 112, No. 4, 1992) has been developed.
As shown in FIG. 7, this electron emission element includes a conductive silicon substrate (cathode substrate) 701 and a silicon layer provided on the silicon substrate 701 and having a conical projection 702 on a surface thereof. The conical projection 702 is formed using microscopic processing technology and acts as an electron emitter section formed of silicon. An anode substrate is provided opposed to the cathode substrate 701 having the electron emitter section. The anode substrate is formed by sequentially depositing a transparent electrode 704 and a phosphor thin film 705, and optionally a metal thin film, on a transparent glass substrate 703. The anode substrate is set up so that a surface thereof having the phosphor thin film 705 faces the electron emitter section.
When the cathode substrate and the anode substrate which are included in a light emission element and are opposed to each other are put in a high vacuum and a prescribed voltage is applied between the cathode substrate and the anode substrate, electrons are emitted from the tip of the electron emitter section into the vacuum. The emitted electrons are accelerated by the applied voltage and reach the phosphor thin film 705. The collision of the electrons with the phosphor thin film 705 causes the phosphor thin film 705 to emit light. The phosohor thin film 703 is allowed to emit light of the three primary colors of red, blue and green or intermediate colors therebetween by changing the materials thereof. The brightness of the light emitted by the phosphor material is controlled by adjusting the voltage a gate electrode 706.
A display device is formed by arranging a plurality of such light emission elements on a plane.
In the case of the above-described conventional electron emission element, the electron emitter section is formed to be conical so that the field intensity of the tip thereof is increased for emitting electrons under low-voltage operation. Accordingly, the current density at the tip is increased.
In addition, since the electron emitter section is formed of silicon which has a lower conductivity than metal, heat is easily generated at the tip during the operation of the element. Accordingly, the tip of the emitter section is vaporized or melted by heat, which increases the radius of curvature of the tip of the emitter section. As a result the electron emission characteristics are deteriorated.
When the electron emission characteristics are thus deteriorated, the brightness of light emitted from the phosphor is lowered. In order to raise the brightness, the operating voltage needs to be raised to recover the, current following though the emitter section. However, since the electric resistance is large at the tip of the emitter section as described above, the amount of heat generated at this section is further increased. Consequently, the electron emission characteristics are acceleratively deteriorated. As a result, the element is destroyed and the desired electron emission is not realized.
As described above, the conventional electron emission element does not allow the operating current to be increased due to the sharp tip configuration of the emitter section, and therefore provides a low brightness of light and a short life, and is inferior in operating stability and reliability. It in very difficult to put such an element into practical use as a display device.